BLACK FLIES (DIPTERA: SIMULIIDAE) OF IRRIGATION SYSTEMS IN SASKATCHEWAN AND ALBERTA

The immature stages of Simulium villatum Zett., venustum Say, meridionale Riley, tuberosum Lund., decorum Walk., bivittatum Mall., griseum Coq., arcticum Mall., and aureum Fries, in decreasing order of abundance, occurred in 79% of 341 collections from 69 irrigation canals and 18 adjacent rivers and streams in irrigated areas of southern Saskatchewan and Alberta in the years 1950 to 1958. Canals less than 4 years old were infested despite a scarcity of aquatic vegetation which formed the main attachment sites. Factors reducing larval abundance included interruption of flow, lack of food and attachment sites, parasites, invertebrate and vertebrate predators, and possibly insecticides from irrigated crops. In some canals up to 100% of the larvae of S. vittatum were infected with nematodes, Hydromermis sp., which prevented pupation. Other larvae were infected with microsporidia. Most canals were dry in the winter. Larvae of only S. vittatum overwintered in two canals which flowed all winter; no viable overwintered eggs of any species were found in any canals. It is presumed that canals are reinfested each spring by gravid females immigrating from permanent breeding sites, mainly in the larger streams and rivers. Although black flies, especially S. vittatum, were occasionally abundant and all are blood feeders, they were generally less of a nuisance than mosquitoes. Leucocytozoon occasionally occurred in turkeys.